I have also heard on Animal Planet to put a wire mesh, like chicken wire, where they dig. You will have to dig up the ground, then put the chicken wire down then gross the grass. The wire is unconfortable to the dogs so they will not want to dig in that area.
This of course does not solve the issue of stopping them digging, it only limits them. My guess a combination of the sand pit for digging and the wire mesh should solve your problem.